There are four different explanations for how the gang acquired Scooby:
Scooby Goes Hollywood - The gang picked Scooby up at a pet store.
Happy Birthday Scooby-Doo & A Pup Named Scooby-Doo - the gang has had Scooby since they were children.
Scooby-Doo! The Mystery Begins - Scooby jumped in Shaggy's window after he saw ghosts.
Scooby Apocalypse (comics) - Shaggy befriends Scooby in a warehouse (The Complex)
In Scooby-Doo and the Witch's Ghost, when Sarah Ravencroft was sucked into the spell book, everything she had cast a spell on (tree, pumpkins, etc.) turned back to normal and the spell was removed. However, the giant turkey stayed giant, leaving a plot hole as to why the spell was not taken off of the turkey.
In Effingham County in 2012, Scooby got a significant amount of write-in votes to be sheriff of the county.